Add support for firefox 28 to os.js

Verification
- [ ] Open FF 27, cmd-opt-k to open web console, copy and paste `data/js/detect/os.js` into the console
- [ ] Running `os_detect.getVersion().ua_version` should return "27.0"
- [ ] Open FF 28, cmd-opt-k to open web console, copy and paste `data/js/detect/os.js` into the console
- [ ] Running `os_detect.getVersion().ua_version` should return "28.0"